Network Working Group                                      D. Harrington
Internet-Draft                                 Huawei Technologies (USA)
Updates: 3411,3412,3414,3417                            J. Schoenwaelder
(if approved)                            International University Bremen
Intended status: Standards Track                        J. Schoenwaelder                        February 5, 2007
Expires: June 16, August 9, 2007                   International University Bremen
                                                       December 13, 2006

 Transport Subsystem for the Simple Network Management Protocol (SNMP)
                        draft-ietf-isms-tmsm-05
                        draft-ietf-isms-tmsm-06

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Copyright Notice

   Copyright (C) The IETF Trust (2006). (2007).

Abstract

   This document describes defines a Transport Subsystem, extending the Simple
   Network Management Protocol (SNMP) architecture defined in RFC 3411.
   This document describes defines a subsystem to contain transport models, Transport Models,
   comparable to other subsystems in the RFC3411 architecture.  As work
   is being done to expand the transport to include secure transport
   such as SSH and TLS, using a subsystem will enable consistent design
   and modularity of such transport models. Transport Models.  This document identifies
   and discusses describes some key aspects that need to be considered for any
   transport model
   Transport Model for SNMP.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3  4
     1.1.  The Internet-Standard Management Framework . . . . . . . .  3  4
     1.2.  Where this Extension Fits  . . . . . . . . . . . . . . . .  4
     1.3.  Conventions  . . . . . . . . . . . . . . . . . . . . . . .  3  6
   2.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . . .  3  6
   3.  Requirements of a Transport Model  . . . . . . . . . . . . . .  6  8
     3.1.  Message Security Requirements  . . . . . . . . . . . . . .  6  8
       3.1.1.  Security Protocol Requirements . . . . . . . . . . . .  6  8
     3.2.  SNMP Requirements  . . . . . . . . . . . . . . . . . . . .  7  9
       3.2.1.  Architectural Modularity Requirements  . . . . . . . .  7  9
       3.2.2.  Access Control Requirements  . . . . . . . . . . . . . 11 13
       3.2.3.  Security Parameter Passing Requirements  . . . . . . . 12 14
       3.2.4.  Separation of Authentication and Authorization . . . . 15
     3.3.  Session Requirements . . . . . . . . . . . . . . . . . . . 14 16
       3.3.1.  Session Establishment Requirements . . . . . . . . . . 14 17
       3.3.2.  Session Maintenance Requirements . . . . . . . . . . . 16 18
       3.3.3.  Message security versus session security . . . . . . . 16 18
   4.  Scenario Diagrams for the Transport Subsystem  . . . . . . . . 17 19
     4.1.  Command Generator or Notification Originator . . . . . . . 17 19
     4.2.  Command Responder  . . . . . . . . . . . . . . . . . . . . 18 21
   5.  Cached Information and References  . . . . . . . . . . . . . . 19 22
     5.1.  securityStateReference . . . . . . . . . . . . . . . . . . 20 23
     5.2.  tmStateReference . . . . . . . . . . . . . . . . . . . . . 21 24
   6.  Abstract Service Interfaces  . . . . . . . . . . . . . . . . . 21 24
     6.1.  Generating an Outgoing SNMP Message  sendMessage ASI  . . . . . . . . . . . 22 . . . . . . . . . . 24
     6.2.  Processing for an  Other Outgoing Message ASIs  . . . . . . . . . . . . 23
     6.3.  Processing an Incoming SNMP Message . . . . . . . 25
     6.3.  The receiveMessage ASI . . . . 23
       6.3.1.  Processing an Incoming Message . . . . . . . . . . . . 23
       6.3.2.  Prepare Data Elements from . . 26
     6.4.  Other Incoming Messages ASIs  . . . . . 23
       6.3.3.  Processing an Incoming Message . . . . . . . . . . . . 24 . . 27
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25 28
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26 29
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 26 29
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26 29
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 26 29
     10.2. Informative References . . . . . . . . . . . . . . . . . . 27 30
   Appendix A.  Parameter Table . . . . . . . . . . . . . . . . . . . 28 31
     A.1.  ParameterList.csv  . . . . . . . . . . . . . . . . . . . . 28 31
   Appendix B.  Why tmStateReference? . . . . . . . . . . . . . . . . 29 33
     B.1.  Define an Abstract Service Interface . . . . . . . . . . . 29 33
     B.2.  Using an Encapsulating Header  . . . . . . . . . . . . . . 30 33
     B.3.  Modifying Existing Fields in an SNMP Message . . . . . . . 30 34
     B.4.  Using a Cache  . . . . . . . . . . . . . . . . . . . . . . 30 34
   Appendix C.  Open Issues . . . . . . . . . . . . . . . . . . . . . 31 34
   Appendix D.  Change Log  . . . . . . . . . . . . . . . . . . . . . 31 35

1.  Introduction

   This document describes defines a Transport Subsystem, extending the Simple
   Network Management Protocol (SNMP) architecture defined in [RFC3411].
   This document identifies and discusses describes some key aspects that need to
   be considered for any transport model Transport Model for SNMP.

1.1.  The Internet-Standard Management Framework

   For a detailed overview of the documents that describe the current
   Internet-Standard Management Framework, please refer to section 7 of
   RFC 3410 [RFC3410].

1.2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in  Where this Extension Fits

   It is expected that readers of this document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Motivation

   There are multiple ways to secure one's home or business, in will have read RFC3410
   and RFC3411, and have a
   continuum of alternatives.  Let's consider three general approaches.
   In understanding of the first approach, functionality
   defined in RFCs 3412-3418.

   The "Transport Subsystem" is an individual could buy a gun, learn to use
   it, additional component for the SNMP
   Engine depicted in RFC3411, section 3.1.

   The following diagram depicts its place in the RFC3411 architecture.:

   +-------------------------------------------------------------------+
   |  SNMP entity                                                      |
   |                                                                   |
   |  +-------------------------------------------------------------+  |
   |  |  SNMP engine (identified by snmpEngineID)                   |  |
   |  |                                                             |  |
   |  |  +------------+                                             |  |
   |  |  | Transport  |                                             |  |
   |  |  | Subsystem  |                                             |  |
   |  |  +------------+                                             |  |
   |  |                                                             |  |
   |  |  +------------+ +------------+ +-----------+ +-----------+  |  |
   |  |  | Dispatcher | | Message    | | Security  | | Access    |  |  |
   |  |  |            | | Processing | | Subsystem | | Control   |  |  |
   |  |  |            | | Subsystem  | |           | | Subsystem |  |  |
   |  |  +------------+ +------------+ +-----------+ +-----------+  |  |
   |  +-------------------------------------------------------------+  |
   |                                                                   |
   |  +-------------------------------------------------------------+  |
   |  |  Application(s)                                             |  |
   |  |                                                             |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  |  | Command     |  | Notification |  | Proxy        |        |  |
   |  |  | Generator   |  | Receiver     |  | Forwarder    |        |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  |                                                             |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  |  | Command     |  | Notification |  | Other        |        |  |
   |  |  | Responder   |  | Originator   |  |              |        |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  +-------------------------------------------------------------+  |
   |                                                                   |
   +-------------------------------------------------------------------+

   The transport mappings defined in RFC3417 do not provide lower-layer
   security functionality, and thus do not provide transport-specific
   security parameters.  This document updates RFC3411 and RFC3417 by
   defining an architectural extension and ASIs that transport mappings
   (models) can use to pass transport-specific security parameters to
   other subsystems, including transport-specific security parameters
   translated into the transport-independent securityName and
   securityLevel.

1.3.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   The key words "must", "must not", "required", "shall", "shall not",
   "should", "should not", "recommended", "may", and "optional" in this
   document are not to be interpreted as described in RFC2119.  They
   will usually, but not always, be used in a context relating to
   compatibility with the RFC3411 architecture or the subsystem defined
   here, but which might have no impact on on-the-wire compatibility.
   These terms are used as guidance for designers of proposed IETF
   models to make the designs compatible with RFC3411 subsystems and
   Abstract Service Interfaces (see section 3.2).  Implementers are free
   to implement differently.  Some usages of these lowercase terms are
   simply normal English usage.

2.  Motivation

   Just as there are multiple ways to secure one's home or business, in
   a continuum of alternatives, there are multiple ways to secure a
   network management protocol.  Let's consider three general
   approaches.

   In the first approach, an individual could sit on your his front porch
   waiting for intruders.  In the second approach, one he could hire an
   employee with a gun, , schedule the employee, position the employee to guard what you want
   he wants protected, hire a second guard to cover if the first gets
   sick, and so on.  In the third approach, you he could hire a security
   company, tell them what
   you want he wants protected, and they could hire
   employees, train them, buy
   the guns, position the guards, schedule the guards, send
   a replacement when a guard cannot make it, etc., thus providing the
   security you want,
   desired security, with no significant effort on your his part other than
   identifying requirements and verifying the quality of the service
   being provided.

   The User-based Security Model (USM) as defined in [RFC3414] largely
   uses the first approach - it provides its own security.  It utilizes
   existing mechanisms (SHA=the gun), (e.g., SHA), but provides all the coordination.
   USM provides for the authentication of a principal, message
   encryption, data integrity checking, timeliness checking, etc.

   USM was designed to be independent of other existing security
   infrastructures.  USM therefore requires a separate principal and key
   management infrastructure.  Operators have reported that deploying
   another principal and key management infrastructure in order to use
   SNMPv3 is a deterrent to deploying SNMPv3.  It is possible but
   difficult to define use
   external mechanisms that to handle the distribution of keys for use by the USM approach.
   USM.  The more important issue is that operators wanted to leverage a
   single user base that wasn't specific to SNMP.

   A solution based on the second approach might use a USM-compliant
   architecture, but combine the authentication mechanism with an
   external mechanism, such as RADIUS [RFC2865], to provide the
   authentication service.  It might be possible to utilize an external
   protocol to encrypt a message, to check timeliness, to check data
   integrity, etc.  It is difficult to cobble together a number of
   subcontracted services and coordinate them however, because it is
   difficult to build solid security bindings between the various
   services, and potential for gaps in the security is significant.

   A solution based on the third approach might utilize one or more
   lower-layer security mechanisms to provide the message-oriented
   security services required.  These would include authentication of
   the sender, encryption, timeliness checking, and data integrity
   checking.  There are a number of IETF standards available or in
   development to address these problems through security layers at the
   transport layer or application layer, among them TLS [RFC4366], SASL
   [RFC4422], and SSH [RFC4251].

   From an operational perspective, it is highly desirable to use
   security mechanisms that can unify the administrative security
   management for SNMPv3, command line interfaces (CLIs) and other
   management interfaces.  The use of security services provided by
   lower layers is the approach commonly used for the CLI, and is also
   the approach being proposed for NETCONF [I-D.ietf-netconf-ssh].

   This document describes a Transport Subsystem extension to the
   RFC3411 architecture.

   +-------------------------------------------------------------------+
   |  SNMP entity                                                      |
   |                                                                   |
   |  +-------------------------------------------------------------+  |
   |  |  SNMP engine (identified by snmpEngineID)                   |  |
   |  |                                                             |  |
   |  |  +------------+                                             |  |
   |  |  | Transport  |                                             |  |
   |  |  | Subsystem  |                                             |  |
   |  |  +------------+                                             |  |
   |  |                                                             |  |
   |  |  +------------+ +------------+ +-----------+ +-----------+  |  |
   |  |  | Dispatcher | | Message    | | Security  | | Access    |  |  |
   |  |  |            | | Processing | | Subsystem | | Control   |  |  |
   |  |  |            | | Subsystem  | |           | | Subsystem |  |  |
   |  |  +------------+ +------------+ +-----------+ +-----------+  |  |
   |  +-------------------------------------------------------------+  |
   |                                                                   |
   |  +-------------------------------------------------------------+  |
   |  |  Application(s)                                             |  |
   |  |                                                             |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  |  | Command     |  | Notification |  | Proxy        |        |  |
   |  |  | Generator   |  | Receiver     |  | Forwarder    |        |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  |                                                             |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  |  | Command     |  | Notification |  | Other        |        |  |
   |  |  | Responder   |  | Originator   |  |              |        |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  +-------------------------------------------------------------+  |
   |                                                                   |
   +-------------------------------------------------------------------+ security services provided by
   lower layers is the approach commonly used for the CLI, and is also
   the approach being proposed for NETCONF [RFC4741].

   This document defines a Transport Subsystem extension to the RFC3411
   architecture based on the third approach.  This extension specifies
   how other lower layer protocols with common security infrastructures
   can be used underneath the SNMP protocol and the desired goal of
   unified administrative security can be met.

   This extension allows security to be provided by an external protocol
   connected to the SNMP engine through an SNMP transport-model Transport Model
   [RFC3417].  Such a transport model Transport Model would then enable the use of
   existing security mechanisms such as (TLS) [RFC4366] or SSH [RFC4251]
   within the RFC3411 architecture.

   There are a number of Internet security protocols and mechanisms that
   are in wide spread use.  Many of them try to provide a generic
   infrastructure to be used by many different application layer
   protocols.  The motivation behind the transport subsystem Transport Subsystem is to
   leverage these protocols where it seems useful.

   There are a number of challenges to be addressed to map the security
   provided by a secure transport into the SNMP architecture so that
   SNMP continues to work without any surprises. provide interoperability with existing
   implementations.  These challenges are
   discussed described in detail in this
   document.  For some key issues, design choices are discussed described that may
   might be made to provide a workable solution that meets operational
   requirements and fits into the SNMP architecture defined in
   [RFC3411].

3.  Requirements of a Transport Model

3.1.  Message Security Requirements

   Transport security protocols SHOULD ideally provide the protection against the
   following message-oriented threats [RFC3411]:

   1.  modification of information
   2.  masquerade
   3.  message stream modification
   4.  disclosure

   According to [RFC3411], it

   These threats are described in section 1.4 of [RFC3411].  It is not
   required to protect against denial of service or traffic analysis. analysis,
   but it should not make those threats significantly worse.

3.1.1.  Security Protocol Requirements

   There are a number of standard protocols that could be proposed as
   possible solutions within the transport subsystem. Transport Subsystem.  Some factors
   should
   SHOULD be considered when selecting a protocol.

   Using a protocol in a manner for which it was not designed has
   numerous problems.  The advertised security characteristics of a
   protocol may might depend on its it being used as designed; when used in
   other ways, it may might not deliver the expected security
   characteristics.  It is recommended that any proposed model include a discussion
   description of the applicability of the transport model. Transport Model.

   A transport model should Transport Model SHOULD require no modifications to the underlying
   protocol.  Modifying the protocol may might change its security
   characteristics in ways that would impact other existing usages.  If
   a change is necessary, the change should be an extension that has no
   impact on the existing usages.  It is recommended that any transport
   model include a discussion of potential impact on other usages of the
   protocol.

   It has been a long-standing requirement that SNMP be able to work
   when the network is unstable, to enable network troubleshooting and
   repair.  The UDP approach has been considered to meet that need well,
   with an assumption that getting small messages through, even if out
   of order, is better than getting no messages through.  There has been
   a long debate about whether UDP actually offers better support than
   TCP when the underlying IP or lower layers are unstable.  There has
   been recent discussion of whether operators actually use SNMP to
   troubleshoot and repair unstable networks.

   There has been discussion of ways SNMP could be extended to better
   support management/monitoring needs when other existing usages.  If
   a network change is running just
   fine.  Use of necessary, the change SHOULD be an extension that has no
   impact on the existing usages.  Any Transport Model SHOULD include a TCP transport, for example, could enable larger
   message sizes and more efficient table retrievals.
   description of potential impact on other usages of the protocol.

   Transport models Models MUST be able to coexist with other transport models,
   and may be designed to utilize either TCP or UDP or SCTP. each other.

3.2.  SNMP Requirements

3.2.1.  Architectural Modularity Requirements

   SNMP version 3 (SNMPv3) is based on a modular architecture (described (defined
   in [RFC3411] section 3) to allow the evolution of the SNMP protocol
   standards over time, and to minimize side effects between subsystems
   when changes are made.

   The RFC3411 architecture includes a security subsystem Security Subsystem for enabling
   different methods of providing security services, a messaging
   subsystem Message
   Processing Subsystem permitting different message versions to be
   handled by a single engine, an application subsystem Applications(s) to support different
   types of application processors, and an access control subsystem Access Control Subsystem for
   allowing multiple approaches to access control.  The RFC3411
   architecture does not include a subsystem for transport models, Transport Models,
   despite the fact there are multiple transport mappings already
   defined for SNMP.  This document addresses the need for a transport subsystem Transport
   Subsystem compatible with the RFC3411 architecture.

   In SNMPv2, there were many problems of side effects between
   subsystems caused by the manipulation of MIB objects, especially
   those related to authentication and authorization, because many of
   the parameters were stored in shared MIB objects, and different
   models and protocols could assign different values to the objects.
   Contributors assumed slightly different shades of meaning depending
   on the models and protocols being used.  As the shared MIB module
   design was modified to accommodate a specific model, other models
   which used the same MIB objects would be broken.

   Abstract Service Interfaces (ASIs) were developed to pass model-
   independent parameters.  The models were required work is being
   done to translate from
   their model-dependent formats into a model-independent format,
   defined using model-independent semantics, which would not impact
   other models.

   Parameters have been provided in expand the ASIs transport to pass model-independent
   information about the authentication that has been provided.  These
   parameters include a model-independent identifier of the security
   "principal", the security model used to perform the authentication,
   and which SNMP-specific security features were applied to the message
   (authentication and/or privacy).

   Parameters have been provided in the ASIs to pass model-independent secure transport address information.  These parameters utilize the
   transportDomain such as SSH
   and TLS, using a subsystem will enable consistent design and transportAddress
   modularity of such Transport Models.

   The design of a transport subsystem must abide this Transport Subsystem accepts the goals of the
   RFC3411 architecture defined in section 1.5 of [RFC3411].  To that end, this
   transport subsystem proposal  This
   Transport Subsystem uses a modular design that will permit
   transport models Transport
   Models to be advanced through the standards process independently of
   other transport models, Transport Models, and independent of other modular SNMP
   components as much as possible.

   Parameters have been added to the ASIs to pass model-independent
   transport address information.

   IETF standards typically require one mandatory to implement solution,
   with the capability of adding new mechanisms in the future.  Part of
   the motivstion motivation of developing transport models Transport Models is to develop support
   for secure transport protocols, such as a transport model Transport Model that
   utilizes the Secure Shell protocol.  Any transport model should Transport Model SHOULD
   define one minimum-compliance security mechanism, preferably one
   which is already widely used such as
   certificates, to secure the transport layer protocol. ensure a basic level of interoperability, but should
   also be able to support additional existing and new mechanisms.

   The Transport Subsystem permits multiple transport protocols to be
   "plugged into" the RFC3411 architecture, supported by corresponding
   transport models,
   Transport Models, including models that are security-aware.

   The RFC3411 architecture,and architecture and the USM Security Subsystem assume that a security model
   Security Model is called by a message-processing model Message Processing Model and will
   perform multiple security functions within the security subsystem. Security Subsystem.  A transport model
   Transport Model that supports a secure transport protocol may might
   perform similar security functions within the transport subsystem. Transport Subsystem.  A transport model
   may
   Transport Model might perform the translation of transport security
   parameters to/from security-model-independent parameters.

   To accommodate this, an implementation-specific cache of transport-
   specific information will be described (not shown), and the ASIs
   for data
   flows between the transport subsystem, Transport Subsystem and the messaging subsystem, Transport Dispatch,
   between the Message Dispatch and the
   security subsystem Message Processing Subsystem,
   and between the Message Processing Subsystem and the Security
   Subsystem will be extended to pass security-model-
   independent values, security-model-independent values.
   New Security Models may also be defined that understand how to work
   with the modified ASIs and the cache.  One such Security Mode, the
   Transport Security Model, is defined in

   The following diagram depicts the SNMPv3 architecture including the
   new Transport Subsystem defined in this document, and a cache of transport-specific information. new Transport
   Security Model defined in [I-D.ietf-isms-transport-security-model].

   +------------------------------+
   |    Network                   |
   +------------------------------+
      ^       ^              ^
      |       |              |
      v       v              v                 (traditional SNMP agent)
   +-------------------------------------------------------------------+
   | +--------------------------------------------------+              |
   | |  Transport Subsystem                             |              |
   | | +-----+ +-----+ +-----+ +-----+       +-------+  |              |
   | | | UDP | | TCP | | SSH | | TLS | . . . | other |  |              |
   | | +-----+ +-----+ +-----+ +-----+       +-------+  |              |
   | +--------------------------------------------------+              |
   |              ^                                                    |
   |              |                                                    |
   | Dispatcher   v                                                    |
   | +-------------------+ +---------------------+  +----------------+ |
   | | Transport         | | Message Processing  |  | Security       | |
   | | Dispatch          | | Subsystem           |  | Subsystem      | |
   | |                   | |     +------------+  |  | +------------+ | |
   | |                   | |  +->| v1MP     *       |<--->| | USM      *        | | |
   | |                   | |  |  +------------+  |  | +------------+ | |
   | |                   | |  |  +------------+  |  | +------------+ | |
   | |                   | |  +->| v2cMP    *      |<--->| | Transport* Transport  | | |
   | | Message           | |  |  +------------+  |  | | Security   | | |
   | | Dispatch    <--------->|  +------------+  |  | | Model      | | |
   | |                   | |  +->| v3MP     *       |<--->| +------------+ | |
   | |                   | |  |  +------------+  |  | +------------+ | |
   | | PDU Dispatch      | |  |  +------------+  |  | | Other    *      | | |
   | +-------------------+ |  +->| otherMP  *    |<--->| | Model(s)   | | |
   |              ^        |     +------------+  |  | +------------+ | |
   |              |        +---------------------+  +----------------+ |
   |              v                                                    |
   |      +-------+-------------------------+---------------+          |
   |      ^                                 ^               ^          |
   |      |                                 |               |          |
   |      v                                 v               v          |
   | +-------------+   +---------+   +--------------+  +-------------+ |
   | |   COMMAND   |   | ACCESS  |   | NOTIFICATION |  |    PROXY    | |
   | |  RESPONDER  |<->| CONTROL |<->|  ORIGINATOR  |  |  FORWARDER  | |
   | | application   +---------+   +--------------+  +-------------+ |
   | |   COMMAND   | applications   | ACCESS  | application   | NOTIFICATION |  | +-------------+   +---------+   +--------------+  +-------------+    PROXY    | |      ^                                 ^
   | |  RESPONDER  |<->| CONTROL |<->|  ORIGINATOR  |  |  FORWARDER  | |      v                                 v
   | | +----------------------------------------------+ application |   |         |             MIB instrumentation   |      SNMP entity applications |  | application | |
   | +-------------+   +---------+   +--------------+  +-------------+ |
   |      ^                                 ^                          |
   |      |                                 |                          |
   |      v                                 v                          |
   | +----------------------------------------------+                  |
   | |             MIB instrumentation              |
   +-------------------------------------------------------------------+

3.2.1.1.  USM and the RFC3411 Architecture

   The following diagrams illustrate the difference in the security
   processing done by the USM model and the security processing
   potentially done by a transport model.

   The USM security model is encapsulated by the messaging model,
   because the messaging model needs to perform the following steps (for
   incoming messages)
   1) decode the ASN.1 (messaging model)
   2) determine the SNMP security model and parameters (messaging model)
   3) decrypt the encrypted portions of the message (security model)
   4) translate parameters to model-independent parameters (security
      model)
   5) determine which application should get the decrypted portions
      (messaging model), and
   6) pass on the decrypted portions with model-independent parameters.

   The USM approach uses SNMP-specific message security and parameters.

3.2.1.2.  Transport Subsystem and the RFC3411 Architecture

   With the Transport Subsystem, the order of the steps may differ and
   may be handled by different subsystems:
   1) decrypt the encrypted portions of the message (transport layer)
   2*)  translate parameters to model-independent parameters (transport
      model)
   3) determine the SNMP security model and parameters (transport model)
   4) decode the ASN.1 (messaging model)
   5) determine which application should get the decrypted portions
      (messaging model)
   7) pass on the decrypted portions with model-independent security
      parameters

   If a message is secured using non-SNMP-specific message security and
   parameters, then the transport model should provide the translation
   from the authenticated identity (e.g., an SSH user name) to the
   securityName in step 3.

3.2.1.3.  Passing Information between Engines

   A secure transport model will establish an encrypted tunnel between
   the transport models of two      SNMP engines.  One transport model
   instance encrypts all messages, entity |
   +-------------------------------------------------------------------+

3.2.1.1.  Processing Differences between USM and the other transport model
   instance decrypts the messages.

   After a transport layer tunnel is established, then SNMP messages can
   conceptually be sent through the tunnel from one SNMP engine to
   another SNMP engine.  Once the tunnel Secure Transport

   USM and secure transports differ is established, multiple SNMP
   messages may be able to be passed through the same tunnel.

3.2.2.  Access Control Requirements

3.2.2.1.  securityName Binding

   For SNMP access control to function properly, security processing
   must establish a securityModel identifier, a securityLevel, order and a
   securityName, which is
   responsibilities within the security model independent identifier for
   a principal.  The message processing subsystem relies on a security
   model, such as USM, to play a role in security that goes beyond
   protecting RFC3411 architecture.  While the message - it provides a mapping between steps
   are the USM-
   specific principal to same, they occur in a security-model independent securityName which
   can different order, and may be used for subsequent processing, such as for access control. done by
   different subsystems.  The securityName MUST be bound to following lists illustrate the mechanism-specific
   authenticated identity, difference
   in the flow and this mapping MUST be done the responsibility for different processing steps for
   incoming messages before the security model passes securityName to the message
   processing model via the processIncoming() ASI.  This translation
   from a mechanism-specific authenticated identity to when using USM and when using a securityName
   MAY be done by secure transport.
   (Note that these lists are simplified for illustrative purposes, and
   do not represent all details of processing.  Transport Models must
   provide the transport model, detailed elements of procedure.)

   With USM and other Security Models, security processing starts when
   the securityname is then
   provided to Message Processing Model decodes portions of the security model to be passed ASN.1 message to
   extract an opaque block of security parameters and header parameters
   that identify which Security Model should process the message processing
   model.

   If the type to
   perform authentication, decryption, timeliness checking, integrity
   checking, and translation of authentication provided by the transport layer (e.g.
   TLS) is considered adequate parameters to model-independent
   parameters.  A secure and/or encrypt transport performs those security functions on
   the message, but
   inadequate to provide before the desired granularity of access control (e.g.
   user-based), then a second authentication (e.g., one provided via ASN.1 is decoded.

   Step 6 cannot occur until after decryption occurs.  Step 6 and beyond
   are the same for USM and a
   RADIUS server) MAY be used secure transport.

3.2.1.1.1.  USM and the RFC3411 Architecture

   1) decode the ASN.1 header (Message Processing Model)
   2) determine the SNMP Security Model and parameters (Message
      Processing Model)
   3) verify securityLevel.  [Security Model]
   4) translate parameters to provide model-independent parameters (Security
      Model)
   5) authenticate and decrypt message.  [Security Model]
   6) determine the authentication identity
   which is bound pduType in the decrypted portions (Message
      Processing Model), and
   7) pass on the decrypted portions with model-independent parameters.

3.2.1.2.  Transport Subsystem and the RFC3411 Architecture

   1) authenticate and decrypt message.  [Transport Model]
   2) translate parameters to model-independent parameters (Transport
      Model)
   3) decode the securityName.  This approach would require a
   good analysis of ASN.1 header (Message Processing Model)
   4) determine the potential for man-in-the-middle attacks or
   masquerade possibilities.

3.2.2.2.  Separation of Authentication SNMP Security Model and Authorization

   A transport model that provides parameters (Message
      Processing Model)
   5) verify securityLevel [Security Model]
   6) determine the pduType in the decrypted portions (Message
      Processing Model), and
   7) pass on the decrypted portions with model-independent security services
      parameters

   If a message is secured using a secure transport layer, then the
   Transport Model should take care provide the translation from the authenticated
   identity (e.g., an SSH user name) to
   not violate the separation of authentication and authorization securityName in the
   RFC3411 architecture.  The isAccessAllowed() primitive is used for
   passing security-model independent parameters step 3.

3.2.1.3.  Passing Information between Engines

   A secure Transport Model will establish an authenticated and/or
   encrypted tunnel between the subsystems Transport Models of two SNMP engines.
   After a transport layer tunnel is established, then SNMP messages can
   be sent through the architecture.

   Mapping of (securityModel, securityName) tunnel from one SNMP engine to an access control policy
   should be handled within the access control subsystem, not other SNMP
   engine.  Transport Models MAY support sending multiple SNMP messages
   through the
   transport or security subsystems, same tunnel.

3.2.2.  Access Control Requirements

   RFC3411 made some design decisions related to be consistent with the
   modularity support of an
   Access Control Subsystem.  These include a securityName and
   securityLevel mapping, the RFC3411 architecture.  This separation was a
   deliberate decision of the SNMPv3 WG, to allow support for
   authentication protocols which did not provide authorization
   capabilities, Authentication and
   Authorization, and to support authorization schemes, such as VACM,
   that do not perform their own authentication.

   An authorization model (in the access control subsystem) MAY require
   authentication by certain securityModels passing of model-independent security
   parameters.

3.2.2.1.  securityName and a minimum securityLevel
   to allow Mapping

   For SNMP access control to the data.

   Transport models that provide secure transport are an enhancement for
   the SNMPv3 privacy function properly, Security Models MUST
   establish a securityLevel and authentication, but they are not a significant
   improvement for the authorization (access control) needs of SNMPv3.
   Only securityName, which is the security-
   model-independent parameters identifier for a principal.  The Message Processing
   Subsystem relies on a Security Model, such as USM, to play a role in
   security that goes beyond protecting the isAccessAllowed()
   primitive [RFC3411] are provided by message - it provides a
   mapping between the transport and security
   subsystems.

   A transport security-model-specific principal to a security-
   model must not specify how the securityModel and independent securityName could which can be dynamically mapped to an access control
   mechanism, used for subsequent
   processing, such as a VACM-style groupName.  The mapping of
   (securityModel, securityName) to a groupName is a VACM-specific
   mechanism for naming an access control policy, control.

   The securityName MUST be mapped from the mechanism-specific
   authenticated identity, and this mapping must be done for tying incoming
   messages before the
   named policy Security Model passes securityName to the addressing capabilities of Message
   Processing Model via the data modeling
   language (e.g.  SMIv2 [RFC2578]), processIncoming ASI.  This translation from
   a mechanism-specific authenticated identity to a securityName might
   be done by the operations supported, Transport Model, and other
   factors.  Providing a binding outside the Access Control subsystem
   might create dependencies that could make it harder to develop
   alternate models of access control, such as one built on UNIX groups
   or Windows domains.  The preferred approach securityName is then provided
   to pass the model-
   independent security parameters Security Model via the isAccessAllowed() ASI, and
   perform the mapping from the model-independent security parameters tmStateReference to
   an authorization-model-dependent access policy within the access
   control model.

   To provide support for protocols which simultaneously send
   information for authentication and authorization, such as RADIUS
   [RFC2865], model-specific authorization information MAY be cached or
   otherwise made available passed to the access control subsystem, e.g., via a
   MIB table similar to
   Message Processing Model.

   If the vacmSecurityToGroupTable, so type of authentication provided by the access
   control subsystem can create an appropriate binding between transport layer (e.g.,
   TLS) is considered adequate to secure and/or encrypt the
   model-independent securityModel and securityName and a model-specific message, but
   inadequate to provide the desired granularity of access control policy.  This may be highly undesirable, however, if
   it creates a dependency between
   (e.g., user-based), then a transport model or second authentication (e.g., one provided
   via a security model
   and an access control model, just as it RADIUS server) MAY be used to provide the authentication
   identity which is undesirable for a
   transport model mapped to create a dependency between an SNMP message
   version and the security provided by securityName.  This approach would
   require a transport model. good analysis of the potential for man-in-the-middle
   attacks or masquerade possibilities.

3.2.3.  Security Parameter Passing Requirements

   RFC3411 section 4 describes primitives to describe the abstract data flows between the various
   subsystems, models and applications within the architecture.
   Abstract Service Interfaces describe the flow of
   data data, passing model-
   independent information between subsystems within an engine.  The ASIs generally pass
   model-independent information.

   Within an engine using a transport model, outgoing SNMP messages are
   passed unencrypted from the message dispatcher to
   RFC3411 architecture has no ASI parameters for passing security
   information between the transport
   model, Transport Subsystem and incoming messages are passed unencrypted from the
   transport model to dispatcher, or
   between the message dispatcher. dispatcher and the Message Processing Model.  This
   document defines or modifies ASIs for this purpose.

   The security parameters include a model-independent identifier of the
   security "principal", "principal" (the securityName), the security model Security Model used to
   perform the authentication, and which SNMP-specific security authentication and privacy
   services were (should be) applied to the message (authentication and/or privacy).

   In the RFC3411 architecture, which reflects the USM security model
   design, the messaging model must (securityLevel).

   A Message Processing Model might unpack SNMP-specific security
   parameters from an incoming message before calling a specific
   security model
   Security Model to authenticate and decrypt an incoming message,
   perform integrity checking, and translate model-specific security security-model-specific
   parameters into model-independent parameters.  When using a secure transport model,
   Transport Model, security parameters MAY might be provided through means
   other than carrying them in the SNMP message.
   The parameters MAY be provided by SNMP applications for outgoing
   messages, and message; the parameters for
   incoming messages MAY might be extracted from the transport layer by transport layer by the
   Transport Model before the message is passed to the Message
   Processing Subsystem.

   This document describes a cache mechanism (see Section 5), into which
   the Transport Model puts information about the transport and security
   parameters applied to a transport connection or an incoming message,
   and a Security Model may extract that information from the cache.  A
   tmStateReference is passed as an extra parameter in the transport model before ASIs of the message
   Transport Subsystem and the Message Processing and Security
   Subsystems, to identify the relevant cache.  This approach of passing
   a model-independent reference is consistent with the
   securityStateReference cache already being passed to around in the message processing subsystem.
   RFC3411 ASIs.

   For outgoing messages, even when a secure transport model Transport Model will
   provide the security services, it is necessary to a Message Processing Model might have an security
   model because it is the security model that
   a Security Model actually creates create the message from its component
   parts.  Whether there are any security services provided by the security model
   Security Model for an outgoing message is
   model-dependent. security-model-dependent.
   For incoming messages, even when a secure transport model Transport Model provides
   security services, a security model is necessary because there Security Model might
   be provide some security
   functionality that can only be provided after the message version is known. or
   other parameters are extracted from the message.

3.2.4.  Separation of Authentication and Authorization

   The message version is determined by RFC3411 architecture defines a separation of authentication and
   authorization (access control), and a Transport Model that provides
   security services should take care to not violate this separation.  A
   Transport Model must not specify how the
   Message Processing model securityModel and passed
   securityName could be dynamically mapped to an access control
   mechanism, such as a VACM-style groupName.

   The RECOMMENDED approach is to pass the model-independent security model
   parameters via the
   processIncoming() ASI. isAccessAllowed ASI, and perform the mapping from
   the model-independent security parameters to an access-control-model-
   dependent policy within the Access Control Model.  The RFC3411 architecture has no
   isAccessAllowed ASI parameters is used for passing the securityModel,
   securityName, and securityLevel parameters that are independent of
   any specific security
   information between model and any specific access control model to
   the Access Control Subsystem.

   The mapping of (securityModel, securityName, securityLevel) to an
   access-control-model-specific policy should be handled within a transport
   specific access control model.  This mapping (a transport model) and should not be done in
   the
   dispatcher, and between Transport or Security Subsystems, to be consistent with the dispatcher and
   modularity of the message processing
   model. RFC3411 architecture.  This document describes separation was a cache mechanism, into which the transport
   model puts information about
   deliberate decision of the transport and security parameters
   applied SNMPv3 WG, to a transport connection or an incoming message, allow support for
   authentication protocols which did not provide authorization (access
   control) capabilities, and a
   security model MAY extract to support authorization schemes, such as
   VACM, that information from do not perform their own authentication.

   The View-based Access Control Model uses the cache.  A
   tmStateReference is passed securityModel and the
   securityName as an extra parameter in inputs to check for access rights.  It determines the ASIs
   groupName as a function of the
   transport subsystem securityModel and securityName.  Providing
   a binding outside the messaging Access Control Subsystem might create
   dependencies that could make it harder to develop alternate models of
   access control, such as one built on UNIX groups or Windows domains.

   To provide support for protocols which simultaneously send
   information for authentication and security subsystems, authorization (access control),
   such as RADIUS [RFC2865], access-control-model-specific information
   might be cached or otherwise made available to
   identify the relevant cache.

   This approach of passing Access Control
   Subsystem, e.g., via a model-independent reference is consistent
   with MIB table similar to the securityStateReference cache already being passed around in
   vacmSecurityToGroupTable, so the RFC3411 ASIs. Access Control Subsystem can create
   an appropriate binding between the access-control-model-independent
   securityModel and securityName and an access-control-model-specific
   policy.  This would be highly undesirable, however, if it creates a
   dependency between a Transport Model or a Security Model and an
   Access Control Model.

3.3.  Session Requirements

   Some secure transports may might have a notion of sessions, while other
   secure transports might provide channels or other session-like thing.
   mechanism.  Throughout this document, the term session is used in a
   broad sense to cover sessions, channels, and session-like things. mechanisms.
   Session refers to an association between two SNMP engines that
   permits the transmission of one or more SNMP messages within the
   lifetime of the session.  How the session is actually established,
   opened, closed, or maintained is specific to a particular transport model. Transport
   Model.

   Sessions are not part of the SNMP architecture described defined in [RFC3411],
   but are considered desirable because the cost of authentication can
   be amortized over potentially many transactions.

   It is important to note that the

   The architecture described defined in [RFC3411] does not include a session
   selector in the Abstract Service Interfaces, and neither is that done
   for the transport subsystem, Transport Subsystem, so an SNMP application cannot has no mechanism
   to select the a session using the ASIs except by passing a unique
   combination of transport type, transport address, transportDomain, transportAddress, securityName,
   securityModel, and securityLevel.

   All  Implementers, of course, might
   provide non-standard mechanisms to select sessions.  The
   transportDomain and transportAddress identify the transport models
   connection to a remote network node; the securityName identifies
   which security principal to communicate with at that address (e.g.,
   different NMS applications), and the securityModel and securityLevel
   might permit selection of different sets of security properties for
   different purposes (e.g., encrypted SETs vs. non-encrypted GETs).

   All Transport Models should discuss the impact of sessions on SNMP
   usage, including how to establish/open a transport session (i.e., how
   it maps to the concepts of session-like things mechanisms of the underlying
   protocol), how to behave when a session cannot be established, how to
   close a session properly, how to behave when a session is closed
   improperly, the session security properties, session establishment
   overhead, and session maintenance overhead.

   To reduce redundancy, this document will discuss describes aspects that are
   expected to be common to all transport model Transport Model sessions.

3.3.1.  Session Establishment Requirements

   SNMP applications must provide the transport type, transport address, transportDomain, transportAddress,
   securityName, securityModel, and securityLevel to be used for a
   session.

   SNMP Applications typically might have no knowledge of whether the session that
   will be used to carry commands was initially established as a
   notification session, or a request-response session, and SHOULD NOT
   make any assumptions based on knowing the direction of the session.
   If an administrator or transport model Transport Model designer wants to
   differentiate a session established for different purposes, such as a
   notification session versus a request-response session, the
   application can use different securityNames or transport addresses
   (e.g., port 161 vs. port 162) for different purposes.

   An SNMP engine containing an application that initiates
   communication, e.g., a Command Generator or Notification Originator,
   MUST
   must be able to attempt to establish a session for delivery if a
   session does not yet exist.  If a session cannot be established then
   the message is discarded.

   Sessions are usually established by the transport model Transport Model when no
   appropriate session is found for an outgoing message, but sessions
   may
   might be established in advance to support features such as
   notifications.  How sessions are established in advance is beyond the
   scope of this document.

   Sessions are initiated by notification originators when there is no
   currently established connection that can be used to send the
   notification.  For a client-server security protocol, this may might
   require provisioning authentication credentials on the agent, either
   statically or dynamically, so the client/agent can successfully
   authenticate to a notification receiver.

   A transport model Transport Model must be able to determine whether a session does or
   does not exist, and must be able to determine which session has the
   appropriate security characteristics (transport type, transport
   address, (transportDomain,
   transportAddress, securityName, securityModel, and securityLevel) for
   an outgoing message.

   A transport model Transport Model implementation MAY reuse an already established
   session with the appropriate transport type, transport address, transportDomain, transportAddress,
   securityName, securityModel, and securityLevel characteristics for
   delivery of a message originated by containing a different type of application pduType than originally
   caused the session to be created.  For example, an implementation
   that has an existing session originally established to receive a
   request may MAY use that session to send an outgoing notification, and may
   MAY use a session that was originally established to send a
   notification to send a request.  Responses are expected to SHOULD be returned using
   the same session that carried the corresponding request message.
   Reuse of sessions is not required for conformance.

   If a session can be reused for a different type of message, pduType, but a receiver is
   not prepared to accept different message types pduTypes over the same session, then
   the message MAY be dropped by the receiver.  This
   may strongly affect the usefulness of session reuse, and transport
   models should define a standard behavior for this circumstance.

3.3.2.  Session Maintenance Requirements

   A transport model Transport Model can tear down sessions as needed.  It may might be
   necessary for some implementations to tear down sessions as the
   result of resource constraints, for example.

   The decision to tear down a session is implementation-dependent.
   While it is possible for an implementation to automatically tear down
   each session once an operation has completed, this is not recommended
   for anticipated performance reasons.  How an implementation
   determines that an operation has completed, including all potential
   error paths, is implementation-dependent.

   The elements of procedure may discuss describe when cached information can be
   discarded, in some circumstances, and the timing of cache cleanup may
   might have security implications, but cache memory management is an
   implementation issue.

   If a transport model Transport Model defines MIB module objects to maintain session
   state information, then the transport model Transport Model MUST describe define what
   happens SHOULD
   happen to the objects when a related session is torn down, since this
   will impact interoperability of the MIB module.

3.3.3.  Message security versus session security

   A transport model Transport Model session is associated with state information that
   is maintained for its lifetime.  This state information allows for
   the application of various security services to multiple messages.
   Cryptographic keys established at the beginning of the session SHOULD
   be used to provide authentication, integrity checking, and encryption
   services for data that is communicated during the session.  The
   cryptographic protocols used to establish keys for a transport model Transport Model
   session SHOULD ensure that fresh new session keys are generated for
   each session.  If each session uses new session keys, then messages
   cannot be replayed from one session to another.  In addition sequence information MAY might be maintained
   in the session which can be used to prevent the replay and reordering
   of messages within a session.  If each session uses new keys, then a
   cross-session replay attack will be unsuccessful; that is, an
   attacker cannot successfully replay on one session a message he
   observed from another session.  A good security protocol will also
   protect against replay attacks _within_ a session; that is, an
   attacker cannot successfully replay a message observed earlier in the
   same session.

   A transport model Transport Model session will typically have a single transport
   type, transport address, transportDomain,
   transportAddress, securityModel, securityName and securityLevel
   associated with it.  If an exchange between communicating engines
   requires a different securityLevel or is on behalf of a different
   securityName, or uses a different securityModel, then another session
   would be needed.  An immediate consequence of this is that
   implementations should SHOULD be able to maintain some reasonable number of
   concurrent sessions.

   For transport models, Transport Models, securityName is typically should be specified during session
   setup, and associated with the session identifier.

   SNMPv3 was designed to support multiple levels of security,
   selectable on a per-message basis by an SNMP application, because because,
   for example, there is not much value in using encryption for a
   Commander Generator to poll for potentially non-sensitive performance
   data on thousands of interfaces every ten minutes; the encryption may
   might add significant overhead to processing of the messages.

   Some transport models MAY Transport Models might support only specific authentication and
   encryption services, such as requiring all messages to be carried
   using both authentication and encryption, regardless of the security
   level requested by an SNMP application.  A transport model MAY Transport Model may
   upgrade the requested security level, i.e. noAuth/noPriv noAuthNoPriv and auth/
   noPriv
   authNoPriv MAY be sent over an authenticated and encrypted session.

4.  Scenario Diagrams for the Transport Subsystem

   RFC3411 section 4.6 provides scenario diagrams to illustrate how an
   outgoing message is created, and how an incoming message is
   processed.  Both diagrams are incomplete, however.  In section 4.6.1,
   the diagram doesn't show the an ASI for sending an SNMP request to the
   network or for receiving an SNMP response message from the network.
   In section 4.6.2, the diagram doesn't illustrate show the interfaces required ASIs to receive an
   SNMP message from the network, or to send an SNMP message to the
   network.

4.1.  Command Generator or Notification Originator

   This diagram from RFC3411 4.6.1 shows how a Command Generator or
   Notification Originator application [RFC3413] requests that a PDU be
   sent, and how the response is returned (asynchronously) to that
   application.

   This document defines a sendMessage ASI to send SNMP messages to the
   network, and a receiveMessage ASI to receive SNMP messages from the
   network.

   Command           Dispatcher               Message           Security
   Generator            |                     Processing           Model
   |                    |                     Model                    |
   |      sendPdu       |                        |                     |
   |------------------->|                        |                     |
   |                    | prepareOutgoingMessage |                     |
   :                    |----------------------->|                     |
   :                    |                        | generateRequestMsg  |
   :                    |                        |-------------------->|
   :                    |                        |                     |
   :                    |                        |<--------------------|
   :                    |                        |                     |
   :                    |<-----------------------|                     |
   :                    |                        |                     |
   :                    |------------------+     |                     |
   :                    | Send SNMP        |     |                     |
   :                    | Request Message  |     |                     |
   :                    | to Network       |     |                     |
   :                    |                  v     |                     |
   :                    :                  :     :                     :
   :                    :                  :     :                     :
   :                    :                  :     :                     :
   :                    |                  |     |                     |
   :                    | Receive SNMP     |     |                     |
   :                    | Response Message |     |                     |
   :                    | from Network     |     |                     |
   :                    |<-----------------+     |                     |
   :                    |                        |                     |
   :                    |   prepareDataElements  |                     |
   :                    |----------------------->|                     |
   :                    |                        | processIncomingMsg  |
   :                    |                        |-------------------->|
   :                    |                        |                     |
   :                    |                        |<--------------------|
   :                    |                        |                     |
   :                    |<-----------------------|                     |
   | processResponsePdu |                        |                     |
   |<-------------------|                        |                     |
   |                    |                        |                     |

4.2.  Command Responder

   This diagram shows how a Command Responder or Notification Receiver
   application registers for handling a pduType, how a PDU is dispatched
   to the application after an SNMP message is received, and how the
   Response is (asynchronously) send sent back to the network.

   This document defines the sendMessage and receiveMessage ASIs for
   this purpose.

   Command               Dispatcher            Message          Security
   Responder                 |                 Processing          Model
   |                         |                 Model                   |
   |                         |                    |                    |
   | registerContextEngineID |                    |                    |
   |------------------------>|                    |                    |
   |<------------------------|              |     |                    |
   |                         | Receive SNMP |     |                    |
   :                         | Message      |     |                    |
   :                         | from Network |     |                    |
   :                         |<-------------+     |                    |
   :                         |                    |                    |
   :                         |prepareDataElements |                    |
   :                         |------------------->|                    |
   :                         |                    | processIncomingMsg |
   :                         |                    |------------------->|
   :                         |                    |                    |
   :                         |                    |<-------------------|
   :                         |                    |                    |
   :                         |<-------------------|                    |
   |     processPdu          |                    |                    |
   |<------------------------|                    |                    |
   |                         |                    |                    |
   :                         :                    :                    :
   :                         :                    :                    :
   |    returnResponsePdu    |                    |                    |
   |------------------------>|                    |                    |
   :                         | prepareResponseMsg |                    |
   :                         |------------------->|                    |
   :                         |                    |generateResponseMsg |
   :                         |                    |------------------->|
   :                         |                    |                    |
   :                         |                    |<-------------------|
   :                         |                    |                    |
   :                         |<-------------------|                    |
   :                         |                    |                    |
   :                         |--------------+     |                    |
   :                         | Send SNMP    |     |                    |
   :                         | Message      |     |                    |
   :                         | to Network   |     |                    |
   :                         |              v     |                    |

5.  Cached Information and References

   The RFC3411 architecture uses caches to store dynamic model-specific
   information, and uses references in the ASIs to indicate in a model-
   independent manner which cached information must flow flows between subsystems.

   There are two levels of state that may might need to be maintained: the
   security state in a request-response pair, and potentially long-term
   state relating to transport and security.

   This state is maintained in caches.  To simplify the elements of
   procedure, the release of state information is not always explicitly
   specified.  As a general rule, if state information is available when
   a message being processed gets discarded, the state related to that
   message should also be discarded, and if state information is
   available when a relationship between engines is severed, such as the
   closing of a transport session, the state information for that
   relationship might also be discarded.

   This document differentiates the tmStateReference from the
   securityStateReference.  This document does not specify an
   implementation strategy, only an abstract discussion description of the data
   that
   must flow flows between subsystems.  An implementation MAY might use one cache
   and one reference to serve both functions, but an implementer must be
   aware of the cache-release issues to prevent the cache from being
   released before a security or transport model Transport Model has had an opportunity
   to extract the information it needs.

5.1.  securityStateReference

   From RFC3411: "For each message received, the Security Model caches
   the state information such that a Response message can be generated
   using the same security information, even if the Local Configuration
   Datastore is altered between the time of the incoming request and the
   outgoing response.

   A Message Processing Model has the responsibility for explicitly
   releasing the cached data if such data is no longer needed. response."  To enable this, an abstract
   securityStateReference data element element, defined in RFC3411 section
   A.1.5, is passed from the Security Model to the Message Processing
   Model.

   The
   cached security data may be implicitly released via the generation of
   a response, or explicitly released by using the stateRelease
   primitive, as described in RFC3411 section 4.5.1."

   The information saved should include the model-independent parameters
   (transportDomain, transportAddress, securityName, securityModel, and
   securityLevel), related security parameters, and other information
   needed to imatch match the response with the request.  The related security
   parameters may include transport-specific security information.

   The Message Processing Model has the responsibility for explicitly
   releasing the securityStateReference when such data is no longer
   needed.  The securityStateReference cached data may be implicitly
   released via the generation of a response, or explicitly released by
   using the stateRelease primitive, ASI, as described defined in RFC 3411 section 4.5.1."

   If the transport model Transport Model connection is closed between the time a
   Request is received and a Response message is being prepared, then
   the Response message MAY be discarded.

5.2.  tmStateReference

   For each message or transport session, information about the message
   security is stored in a cache, which may inlcude include model- and
   mechanism-specific parameters.  The tmStateReference is passed
   between subsystems to provide a handle for the cache.  A transport
   model Transport
   Model may store transport-specific parameters in the cache for
   subsequent usage.  Since the contents of a cache are meaningful only
   within an implementation, and not on-the-wire, the format of the
   cache is implementation-specific.

   The state referenced by tmStateReference may might be saved in a Local
   Configuration Datastore (LCD) to make it available across multiple
   messages, as compared to securityStateReference which is designed to
   be saved only for the life of a request-response pair of messages.
   It is expected that an LCD will allow lookup based on the combination
   of transportDomain, transportAddress, securityName, securityModel,
   and securityLevel, and that the cache contain these values to
   reference entries in the LCD.

6.  Abstract Service Interfaces

   Abstract service interfaces have been defined by RFC 3411 to describe
   the conceptual data flows between the various subsystems within an
   SNMP entity.

   To simplify entity, and to help keep the elements subsystems independent of procedure, each
   other except for the common parameters.

   This document follows the example of RFC3411 regarding the release of
   state information, and regarding error indications.

   1) The release of state information is not always explicitly specified.
   specified in a transport model.  As a general rule, if state
   information is available when a message gets discarded, the
   message-state message-
   state information should also be released, and if state information
   is available when a session is closed, the session state information
   should also be released.

   2) An error indication in statusInformation may return include an OID and
   value for an incremented counter and a value for securityLevel, and
   values for contextEngineID and contextName for the counter, and the
   securityStateReference if the information is available at the point where is available at the point
   where the error is detected.

6.1.  sendMessage ASI

   The sendMessage ASI is used to pass a message from the Dispatcher to
   the appropriate Transport Model for sending.

   If present and valid, the tmStateReference refers to a cache
   containing transport-model-specific parameters for the transport and
   transport security.  How the information in the error cache is
   detected.

6.1.  Generating an Outgoing SNMP Message used is
   transport-model-dependent and implementation-dependent.  How a
   tmStateReference is determined to be present and valid is
   implementation-dependent.

   This section describes the procedure followed by an RFC3411-
   compatible system whenever may sound underspecified, but keep in mind that a transport
   model might be something like SNMP over UDP over IPv6, where no
   security is provided, so it generates might have no mechanisms for utilizing a
   securityName and securityLevel.

   statusInformation =
   sendMessage(
   IN   destTransportDomain           -- transport domain to be used
   IN   destTransportAddress          -- transport address to be used
   IN   outgoingMessage               -- the message containing a
   management operation (such to send
   IN   outgoingMessageLength         -- its length
   IN   tmStateReference              -- reference to transport state
    )

6.2.  Other Outgoing ASIs

   A tmStateReference parameter has been added to the
   prepareOutgoingMessage, generateRequestMsg, and generateResponseMsg
   ASIs as a request, a response, a notification,
   or a report) on behalf of a user. an OUT parameter.

   statusInformation =          -- success or errorIndication
   prepareOutgoingMessage(
   IN  transportDomain          -- transport domain to be used
   IN  transportAddress         -- transport address to be used
   IN  messageProcessingModel   -- typically, SNMP version
   IN  securityModel            -- Security Model to use
   IN  securityName             -- on behalf of this principal
   IN  securityLevel            -- Level of Security requested
   IN  contextEngineID          -- data from/at this entity
   IN  contextName              -- data from/in this context
   IN  pduVersion               -- the version of the PDU
   IN  PDU                      -- SNMP Protocol Data Unit
   IN  expectResponse           -- TRUE or FALSE
   IN  sendPduHandle            -- the handle for matching
                                   incoming responses
   OUT  destTransportDomain     -- destination transport domain
   OUT  destTransportAddress    -- destination transport address
   OUT  outgoingMessage         -- the message to send
   OUT  outgoingMessageLength   -- its length
   OUT  tmStateReference        -- (NEW) reference to transport state
               )

   Note that tmStateReference has been added to this ASI.
   The IN parameters tmStateReference parameter of generateRequestMsg or
   generateResponseMsg is passed in the prepareOutgoingMessage() ASI are used to
   pass information from the dispatcher (from return parameters of the application subsystem)
   Security Subsystem to the message processing subsystem.

   The abstract service primitive from a Message Processing Model to Subsystem.  If a cache
   exists for a session identifiable from transportDomain,
   transportAddress, securityModel, securityName, and securityLevel,
   then an appropriate Security Model might create a tmStateReference to generate
   the components cache and pass that as an OUT parameter.

   If one does not exist, the Security Model might create a cache
   referenced by tmStateReference.  This information might include
   transportDomain, transportAddress, the securityModel, the
   securityLevel, and the securityName, plus any model or mechanism-
   specific details.  The contents of the cache may be incomplete until
   the Transport Model has established a Request message session.  What information is
   passed, and how this information is
   generateRequestMsg(). determined, is implementation and
   security-model-specific.

   The abstract service primitive prepareOutgoingMessage ASI passes tmStateReference from a the
   Message Processing Model to a
   Security Model Subsystem to generate the components of a Response message is
   generateResponseMsg().

   Upon completion of processing, the Security Model returns
   statusInformation.  If the process was successful, dispatcher.  How or if the completed
   message is returned.  If
   Message Processing Subsystem modifies or utilizes the process was not successful, then an
   errorIndication is returned.

   The OUT parameters contents of the prepareOutgoingMessage() ASI are used to
   pass information from the
   cache is message-processing-model-specific.

   This may sound underspecified, but keep in mind that a message
   processing model might have access to all the dispatcher information from the
   cache and on to from the transport model:

6.2.  Processing for an Outgoing Message

   The sendMessage ASI is used message, and have no need to pass call a message from the Dispatcher Security Model
   to
   the appropriate transport model for sending.

   statusInformation =
   sendMessage(
   IN   destTransportDomain           -- transport domain do any processing; an application might choose a Security Model
   such as USM to be used
   IN   destTransportAddress          -- authenticate and secure the SNMP message, but also
   utilize a secure transport address such as that provided by the SSH Transport
   Model to be used
   IN   outgoingMessage               -- send the message to send
   IN   outgoingMessageLength         -- its length
   IN   tmStateReference              -- reference to transport state
    ) destination.

6.3.  Processing an Incoming SNMP Message

6.3.1.  Processing an Incoming Message  The receiveMessage ASI

   If one does not exist, the Transport Model will need to might create an
   entry in a Local Configuration Datastore cache
   referenced by tmStateReference.  This  If present, this information will might
   include transportDomain, transportAddress, the securityModel, the securityLevel, and
   securityName, plus model or mechanism-specific details.  How this
   information is determined is implementation and transport-model-
   specific.

   This may sound underspecified, but keep in mind that a transport
   model might be something like SNMP over UDP over IPv6, where no
   security is provided, so it might have no mechanisms for determining
   a securityName and securityLevel.

   The Transport Model does not know the
   securityName, plus any model or mechanism-specific details.  How securityModel for an incoming
   message; this
   information is will be determined is model-specific. by the Message Processing Model in a
   message-processing-model-dependent manner.

   The recvMessage receiveMessage ASI is used to pass a message from the transport
   subsystem Transport
   Subsystem to the Dispatcher.

   statusInformation =
   recvMessage(
   receiveMessage(
   IN   transportDomain               -- origin transport domain
   IN   transportAddress              -- origin transport address
   IN   incomingMessage               -- the message received
   IN   incomingMessageLength         -- its length
   IN   tmStateReference              -- reference to transport state
    )

6.3.2.  Prepare Data Elements from

6.4.  Other Incoming Messages

   The abstract service primitive from ASIs

   To support the Transport Subsystem, the tmStateReference is added to
   the prepareDataElements ASI (from the Dispatcher to a the Message
   Processing Subsystem), and to the processIncomingMsg ASI (from the
   Message Processing Subsystem to the Security Model for Subsystem).  How
   or if a received message is: Message Processing Model or Security Model uses
   tmStateReference is message-processing-model-dependent and security-
   model-dependent.

   result =                       -- SUCCESS or errorIndication
   prepareDataElements(
   IN   transportDomain           -- origin transport domain
   IN   transportAddress          -- origin transport address
   IN   wholeMsg                  -- as received from the network
   IN   wholeMsgLength            -- as received from the network
   IN   tmStateReference          -- (NEW) from the transport model Transport Model
   OUT  messageProcessingModel    -- typically, SNMP version
   OUT  securityModel             -- Security Model to use
   OUT  securityName              -- on behalf of this principal
   OUT  securityLevel             -- Level of Security requested
   OUT  contextEngineID           -- data from/at this entity
   OUT  contextName               -- data from/in this context
   OUT  pduVersion                -- the version of the PDU
   OUT  PDU                       -- SNMP Protocol Data Unit
   OUT  pduType                   -- SNMP PDU type
   OUT  sendPduHandle             -- handle for matched request
   OUT  maxSizeResponseScopedPDU  -- maximum size sender can accept
   OUT  statusInformation         -- success or errorIndication
                                  -- error counter OID/value if error
   OUT  stateReference            -- reference to state information
                                  -- to be used for possible Response
   )

   Note that tmStateReference has been added to this ASI.

6.3.3.  Processing an Incoming Message

   This section describes the procedure followed by the Security Model
   whenever it receives an incoming message containing a management
   operation on behalf of a user from a Message Processing model.

   The Message Processing Model extracts some information from the
   wholeMsg.  The abstract service primitive from a Message Processing
   Model to the Security Subsystem for a received message is:
   statusInformation =  -- errorIndication or success
                            -- error counter OID/value if error
   processIncomingMsg(
   IN   messageProcessingModel    -- typically, SNMP version
   IN   maxMessageSize            -- of the sending SNMP entity
   IN   securityParameters        -- for the received message
   IN   securityModel             -- for the received message
   IN   securityLevel             -- Level of Security
   IN   wholeMsg                  -- as received on the wire
   IN   wholeMsgLength            -- length as received on the wire
   IN   tmStateReference          -- (NEW) from the transport model Transport Model
   OUT  securityEngineID          -- authoritative SNMP entity
   OUT  securityName              -- identification of the principal
   OUT  scopedPDU,                -- message (plaintext) payload
   OUT  maxSizeResponseScopedPDU  -- maximum size sender can handle
   OUT  securityStateReference    -- reference to security state
    )                         -- information, needed for response

   1)

   The securityEngineID tmStateReference parameter of prepareDataElements is set passed from
   the dispatcher to a value in a model-specific manner.
   If the securityEngineID Message Processing Subsystem.  How or if the
   Message Processing Subsystem modifies or utilizes the contents of the
   cache is not utilized by message-processing-model-specific.

   The processIncomingMessage ASI passes tmStateReference from the specific model, then
   it should be set
   Message Processing Subsystem to the local snmpEngineID, to satisfy Security Subsystem.

   If tmStateReference is present and valid, an appropriate Security
   Model might utilize the SNMPv3
   message processing model information in RFC 3412 section 7.2 13a).

   2) Extract the value of securityName from cache.  How or if the Local Configuration
   Datastore entry referenced by tmStateReference.

   3) The scopedPDU component is extracted from
   Security Subsystem utilizes the information in the wholeMsg.

   4) The maxSizeResponseScopedPDU cache is calculated. security-
   model-specific.

   This is the maximum
   size allowed for a scopedPDU for a possible Response message.

   5) The security data is cached as cachedSecurityData, so may sound underspecified, but keep in mind that a
   possible response to this message can
   processing model might have access to all the information from the
   cache and will use from the same security
   parameters.  Then securityStateReference is set for subsequent
   reference message, and have no need to this cached data.

   6) call a Security Model
   to do any processing.  The statusInformation Message Processing Model might determine
   that the USM Security Model is set specified in an SNMPv3 message header;
   the USM Security Model has no need of values in the tmStateReference
   cache to success authenticate and a return is made to
   the calling module passing back secure the OUT parameters SNMP message, but an application
   might have chosen to use a secure transport such as specified in that provided by
   the processIncomingMsg primitive. SSH Transport Model to send the message to its destination.

7.  Security Considerations

   This document describes defines an architectural approach that would permit permits SNMP to
   utilize transport layer security services.  Each proposed
   transport model Transport
   Model should discuss the security considerations of the
   transport model. Transport
   Model.

   It is considered desirable by some industry segments that SNMP
   transport models
   Transport Models should utilize transport layer security that
   addresses perfect forward secrecy at least for encryption keys.
   Perfect forward secrecy guarantees that compromise of long term
   secret keys does not result in disclosure of past session keys.  The
   editors recommend that each  Each
   proposed transport model Transport Model should include a discussion in its security
   considerations of whether perfect forward security is appropriate for
   the transport model. Transport Model.

   Since the cache and LCD will contain security-related parameters,
   they
   implementers should be kept store this information (in memory or in
   persistent storage) in protected storage. a manner to protect it from unauthorized
   disclosure and/or modification.

   Care must be taken to ensure that a SNMP engine is sending packets
   out over a transport using credentials that are legal for that engine
   to use on behalf of that user.  Otherwise an engine that has multiple
   transports open might be "tricked" into sending a message through the
   wrong transport.

8.  IANA Considerations

   This document requires no action by IANA.

9.  Acknowledgments

   The Integrated Security for SNMP WG would like to thank the following
   people for their contributions to the process:

   The authors of submitted security model Security Model proposals: Chris Elliot, Wes
   Hardaker, Dave David Harrington, Keith McCloghrie, Kaushik Narayan, Dave David
   Perkins, Joseph Salowey, and Juergen Schoenwaelder.

   The members of the Protocol Evaluation Team: Uri Blumenthal,
   Lakshminath Dondeti, Randy Presuhn, and Eric Rescorla.

   WG members who committed to and performed detailed reviews: Jeffrey
   Hutzelman

10.  References

10.1.  Normative References

   [RFC2119]                                 Bradner, S., "Key words for
                                             use in RFCs to Indicate
                                             Requirement Levels", BCP 14,
                           RFC 2119, March 1997.

   [RFC2578]               McCloghrie, K., Ed., Perkins, D., Ed., and J.
                           Schoenwaelder, Ed., "Structure of Management
                           Information Version 2 (SMIv2)", STD 58, Levels",
                                             BCP 14, RFC 2578, April 1999. 2119,
                                             March 1997.

   [RFC3411]                                 Harrington, D., Presuhn,
                                             R., and B. Wijnen, "An
                                             Architecture for Describing
                                             Simple Network Management
                                             Protocol (SNMP) Management
                                             Frameworks", STD 62,
                                             RFC 3411, December 2002.

   [RFC3412]                                 Case, J., Harrington, D.,
                                             Presuhn, R., and B. Wijnen,
                                             "Message Processing and
                                             Dispatching for the Simple
                                             Network Management Protocol
                                             (SNMP)", STD 62, RFC 3412,
                                             December 2002.

   [RFC3414]                                 Blumenthal, U. and B.
                                             Wijnen, "User-based
                                             Security Model (USM) for
                                             version 3 of the Simple
                                             Network Management Protocol
                                             (SNMPv3)", STD 62,
                                             RFC 3414, December 2002.

   [RFC3417]                                 Presuhn, R., "Transport
                                             Mappings for the Simple
                                             Network Management Protocol
                                             (SNMP)", STD 62, RFC 3417,
                                             December 2002.

10.2.  Informative References

   [RFC2865]                                 Rigney, C., Willens, S.,
                                             Rubens, A., and W. Simpson,
                                             "Remote Authentication Dial
                                             In User Service (RADIUS)",
                                             RFC 2865, June 2000.

   [RFC3410]                                 Case, J., Mundy, R.,
                                             Partain, D., and B.
                                             Stewart, "Introduction and
                                             Applicability Statements
                                             for Internet-Standard
                                             Management Framework",
                                             RFC 3410, December 2002.

   [RFC3413]                                 Levi, D., Meyer, P., and B.
                                             Stewart, "Simple Network
                                             Management Protocol (SNMP)
                                             Applications", STD 62,
                                             RFC 3413, December 2002.

   [RFC4366]                                 Blake-Wilson, S., Nystrom,
                                             M., Hopwood, D., Mikkelsen,
                                             J., and T. Wright,
                                             "Transport Layer Security
                                             (TLS) Extensions",
                                             RFC 4366, April 2006.

   [RFC4422]                                 Melnikov, A. and K.
                                             Zeilenga, "Simple
                                             Authentication and Security
                                             Layer (SASL)", RFC 4422,
                                             June 2006.

   [RFC4251]                                 Ylonen, T. and C. Lonvick,
                                             "The Secure Shell (SSH)
                                             Protocol Architecture",
                                             RFC 4251, January 2006.

   [I-D.ietf-netconf-ssh]  Wasserman, M. and T. Goddard, "Using the
                           NETCONF

   [RFC4741]                                 Enns, R., "NETCONF
                                             Configuration Protocol over Secure
                           Shell (SSH)", draft-ietf-netconf-ssh-06 Protocol",
                                             RFC 4741, December 2006.

   [I-D.ietf-isms-transport-security-model]  Harrington, D., "Transport
                                             Security Model for SNMP", d
                                             raft-ietf-isms-transport-
                                             security-model-02 (work in
                                             progress), March 2006. January 2007.

Appendix A.  Parameter Table

   Following is a CSV Comma-separated-values (CSV) formatted matrix useful
   for tracking data flows into and out of the dispatcher, transport, message, Transport,
   Message Processing, and security
   subsystems. Security Subsystems.  This will be of most
   use to designers of models, to understand what information is
   available at which points in the processing, following the RFC3411
   architecture (and this subsystem).  Import this into your favorite
   spreadsheet or other CSV compatible application.  You will need to
   remove lines feeds from the second, third, and fourth lines, which
   needed to be wrapped to fit into RFC limits. line lengths.

A.1.  ParameterList.csv

   ,Dispatcher,,,,Messaging,,,Security,,,Transport,
   ,sendPDU,returnResponse,processPDU,processResponse,

   prepareOutgoingMessage,prepareResponseMessage,prepareDataElements,

   generateRequest,processIncoming,generateResponse,

   sendMessage,recvMessage

   sendMessage,receiveMessage

   transportDomain,In,,,,In,,In,,,,,In

   transportAddress,In,,,,In,,In,,,,,In

   destTransportDomain,,,,,Out,Out,,,,,In,

   destTransportAddress,,,,,Out,Out,,,,,In,

   messageProcessingModel,In,In,In,In,In,In,Out,In,In,In,,

   securityModel,In,In,In,In,In,In,Out,In,In,In,,

   securityName,In,In,In,In,In,In,Out,In,Out,In,,

   securityLevel,In,In,In,In,In,In,Out,In,In,In,,

   contextEngineID,In,In,In,In,In,In,Out,,,,,

   contextName,In,In,In,In,In,In,Out,,,,,

   expectResponse,In,,,,In,,,,,,,

   PDU,In,In,In,In,In,In,Out,,,,,

   pduVersion,In,In,In,In,In,In,Out,,,,,

   statusInfo,Out,In,,In,,In,Out,Out,Out,Out,,

   errorIndication,Out,Out,,,,,Out,,,,,

   sendPduHandle,Out,,,In,In,,Out,,,,,

   maxSizeResponsePDU,,In,In,,,In,Out,,Out,,,

   stateReference,,In,In,,,In,Out,,,,,

   wholeMessage,,,,,Out,Out,In,Out,In,Out,In,In

   messageLength,,,,,Out,Out,In,Out,In,Out,In,In
   maxMessageSize,,,,,,,,In,In,In,,

   globalData,,,,,,,,In,,In,,

   securityEngineID,,,,,,,,In,Out,In,,

   scopedPDU,,,,,,,,In,Out,In,,

   securityParameters,,,,,,,,Out,In,Out,,

   securityStateReference,,,,,,,,,Out,In,,

   pduType,,,,,,,Out,,,,,

   tmStateReference,,,,,Out,Out,In,,In,,In,In

Appendix B.  Why tmStateReference?

   This appendix considers why a cache-based approach was selected for
   passing parameters.  This section may be removed from subsequent
   revisions of the document.

   There are four approaches that could be used for passing information
   between the Transport Model and an a Security Model.

   1.  one could define an ASI to supplement the existing ASIs, or
   2.  one could add a header to encapsulate the SNMP message,
   3.  one could utilize fields already defined in the existing SNMPv3
       message, or
   4.  one could pass the information in an implementation-specific
       cache or via a MIB module.

B.1.  Define an Abstract Service Interface

   Abstract Service Interfaces (ASIs) [RFC3411] are defined by a set of primitives
   that specify the services provided and the abstract data elements
   that are to be passed when the services are invoked.  Defining
   additional ASIs to pass the security and transport information from
   the transport subsystem Transport Subsystem to security subsystem Security Subsystem has the advantage of
   being consistent with existing RFC3411/3412 practice, and helps to
   ensure that any transport model Transport Model proposals pass the necessary data,
   and do not cause side effects by creating model-
   specific model-specific dependencies
   between itself and other models or other subsystems other than those
   that are clearly defined by an ASI.

B.2.  Using an Encapsulating Header

   A header could encapsulate the SNMP message to pass necessary
   information from the Transport Model to the dispatcher and then to a
   messaging security model.
   Message Processing Model.  The message header would be included in
   the wholeMessage ASI parameter, and would be removed by a
   corresponding messaging model. Message Processing Model.  This would imply the (one
   and only) messaging dispatcher would need to be modified to determine
   which SNMP message version was involved, and a new message processing model Message Processing
   Model would need to be developed that knew how to extract the header
   from the message and pass it to the Security Model.

B.3.  Modifying Existing Fields in an SNMP Message

   [RFC3412] describes defines the SNMPv3 message, which contains fields to pass
   security related parameters.  The transport subsystem Transport Subsystem could use these
   fields in an SNMPv3 message, or comparable fields in other message
   formats to pass information between transport models Transport Models in different
   SNMP engines, and to pass information between a transport model Transport Model and a
   corresponding messaging security model. Message Processing Model.

   If the fields in an incoming SNMPv3 message are changed by the
   Transport Model before passing it to the Security Model, then the
   Transport Model will need to decode the ASN.1 message, modify the
   fields, and re-encode the message in ASN.1 before passing the message
   on to the message dispatcher or to the transport layer.  This would
   require an intimate knowledge of the message format and message
   versions so the Transport Model knew which fields could be modified.
   This would seriously violate the modularity of the architecture.

B.4.  Using a Cache

   This document describes a cache, into which the Transport Model puts
   information about the security applied to an incoming message, and a
   Security Model can extract that information from the cache.  Given
   that there may might be multiple TM-security caches, a tmStateReference
   is passed as an extra parameter in the ASIs between the transport
   subsystem Transport
   Subsystem and the security subsystem, Security Subsystem, so the Security Model knows
   which cache of information to consult.

   This approach does create dependencies between a specific Transport
   Model and a corresponding specific Security Model.  However, the
   approach of passing a model-independent reference to a model-
   dependent cache is consistent with the securityStateReference already
   being passed around in the RFC3411 ASIs.

Appendix C.  Open Issues

   NOTE to RFC editor: If this section is empty, then please remove this
   open issues section before publishing this document as an RFC.  (If
   it is not empty, please send it back to the editor to resolve.

   o  MUST responses go back on the same session?
   o  How should we describe the case where a management system wants to
      keep session info available for inspection after a session has
      closed? see "Abstract Service Interfaces"
   o  Do Informs work correctly?
   o  How does a Transport Model know whether a message is a
      notification or a request/response?
   o  cache contents - do we define this?

Appendix D.  Change Log

   NOTE to RFC editor: Please remove this change log before publishing
   this document as an RFC.

   Changes from revision -05- to -06-

      mostly editorial changes
      removed some paragraphs considered unnecessary
      added Updates to header
      modified some text to get the security details right
      modified text re: ASIs so they are not API-like
      cleaned up some diagrams
      cleaned up RFC2119 language
      added section numbers to citations to RFC3411
      removed gun for political correctness

   Changes from revision -04- to -05-

      removed all objects from the MIB module.
      changed document status to "Standard" rather than the xml2rfc
      default of informational.

      changed mention of MD5 to SHA
      moved addressing style to TDomain and TAddress
      modified the diagrams as requested
      removed the "layered stack" diagrams that compared USM and a
      Transport Model processing
      removed discussion of speculative features that might exist in
      future transport models Transport Models
      removed openSession() openSession and closeSession() closeSession ASIs, since those are
      model-dependent model-
      dependent
      removed the MIB module
      removed the MIB boilerplate into intro (this memo defines a SMIv2 MIB
      ...)
      removed IANA considerations related to the now-gone MIB module
      removed security considerations related to the MIB module
      removed references needed for the MIB module
      changed recvMessage receiveMessage ASI to use origin transport domain/address
      updated Parameter CSV appendix
   Changes from revision -03- to -04-

      changed title from Transport Mapping Security Model Architectural
      Extension to Transport Subsystem
      modified the abstract and introduction
      changed TMSM to TMS
      changed MPSP to simply Security Model
      changed SMSP to simply Security Model
      changed TMSP to Transport Model
      removed MPSP and TMSP and SMSP from Acronyms section
      modified diagrams
      removed most references to dispatcher functionality
      worked to remove dependencies between transport and security
      models.
      defined snmpTransportModel enumeration similar to
      snmpSecurityModel, etc.
      eliminated all reference to SNMPv3 msgXXXX fields
      changed tmSessionReference back to tmStateReference

   Changes from revision -02- to -03-

   o  removed session table from MIB module
   o  removed sessionID from ASIs
   o  reorganized to put ASI discussions in EOP section, as was done in
      SSHSM
   o  changed user auth to client auth
   o  changed tmStateReference to tmSessionReference
   o  modified document to meet consensus positions published by JS
   o
      *  authoritative is model-specific
      *  msgSecurityParameters usage is model-specific
      *  msgFlags vs. securityLevel is model/implementation-specific
      *  notifications must be able to cause creation of a session
      *  security considerations must be model-specific
      *  TDomain and TAddress are model-specific
      *  MPSP changed to SMSP (Security model Model security processing)

   Changes from revision -01- to -02-

   o  wrote text for session establishment requirements section.
   o  wrote text for session maintenance requirements section.
   o  removed section on relation to SNMPv2-MIB
   o  updated MIB module to pass smilint
   o  Added Structure of the MIB module, and other expected MIB-related
      sections.
   o  updated author address
   o  corrected spelling
   o  removed msgFlags appendix
   o  Removed section on implementation considerations.
   o  started modifying the security boilerplate to address TMS and MIB
      security issues
   o  reorganized slightly to better separate requirements from proposed
      solution.  This probably needs additional work.
   o  removed section with sample protocols and sample
      tmSessionReference.
   o  Added section for acronyms
   o  moved section comparing parameter passing techniques to appendix.
   o  Removed section on notification requirements.

   Changes from revision -00-
   o  changed SSH references from I-Ds to RFCs
   o  removed parameters from tmSessionReference for DTLS that revealed
      lower layer info.
   o  Added TMS-MIB module
   o  Added Internet-Standard Management Framework boilerplate
   o  Added Structure of the MIB Module
   o  Added MIB security considerations boilerplate (to be completed)
   o  Added IANA Considerations
   o  Added ASI Parameter table
   o  Added discussion of Sessions
   o  Added Open issues and Change Log
   o  Rearranged sections

Authors' Addresses

   David Harrington
   Huawei Technologies (USA)
   1700 Alma Dr. Suite 100
   Plano, TX 75075
   USA

   Phone: +1 603 436 8634
   EMail: dharrington@huawei.com
   Juergen Schoenwaelder
   International University Bremen
   Campus Ring 1
   28725 Bremen
   Germany

   Phone: +49 421 200-3587
   EMail: j.schoenwaelder@iu-bremen.de

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